Abstract / Statement of Work: The vestibular system plays a critical role in detection of head movements and orientation with respect to gravity and is essential for normal postural control. Due to their anatomical proximity to the cochlea, the otolith organs are exposed to sound pressure and are at risk for noise overstimulation, which may contribute to vestibular dysfunction. Clinical reports suggest a link between noise-induced hearing loss and balance problems, particularly in older individuals, but the structural and physiological basis for this linkage is not well understood, and characterization of fall risk in animal models is lacking. It has been established that irregular vestibular afferents exhibit enhanced sensitivity to acceleration. (Goldberg & Fernandez, 1971; For Review: Goldberg et al., 1985). The accuracy of the vestibulo-ocular reflex during high frequency angular accelerations is dependent on irregular vestibular afferents (Kim et al., 2011; Hullar et al., 2005). It is also likely that irregularly discharging, phasic afferents play an important role initiating postural compensation for abrupt changes in head or body position (Peterson et al., 1988; Schor et al., 1988; Bilotto et al., 1982; Wilson et al., 1979). Finally, it has been established that irregular afferents project to secondary vestibular neurons that project to the spinal cord (e.g., Boyle et al., 1992). Although deficits in control of head and body posture may not be obvious during sustained movements, deficits may become apparent when sudden perturbations require rapid resets of center of gravity (COG) or head position in space. Such perturbations may naturally occur to avoid obstacles in one?s path or regain postural stability after a slip, abrupt turn, or unexpected change in heading direction. The goal of this proposal is to characterize fall risk in rodents with noise induced vestibular insults that preferentially impact irregularly firing afferents. Vestibular injury will be induced by exposing rats to 120 decibels, 1.5 kilohertz- centered 3 octave band noise for six hours. Fall risk will be determined through two aims. First, head stability during abrupt perturbations will be measured before and after noise exposure. Second, rats? ability to traverse a narrow balance beam will be compared before and after noise exposure. On random trials the beam?s orientation will be abruptly perturbed to shift the rat?s COG. In order to link changes in head and body stability with peripheral vestibular injury, vestibular short-latency evoked potentials (VsEPs) will be measured before and at critical time points following noise exposure. The VsEP amplitude is a useful metric related to the extent of the peripheral vestibular lesion. Following final measurements, rats will be sacrificed, and vestibular end organs will be stained with antibodies that allow us to differentiate between calyx-only and dimorphic afferents and to quantify loss or injury to their calyx endings. Based on available evidence and our preliminary data, we propose that noise exposure preferentially damages irregular vestibular afferents, resulting in reduced ability to react to abrupt perturbations of the head in space.